Method for producing fluorine-containing silica glass

The present disclosure relates to a method for producing fluorine-containing silica glass. The present application claims priority from Japanese Patent Application No. 2020-162357 filed on Sep. 28, 2020, the entire content of which is incorporated herein by reference.

PTLs 1 to 3 disclose methods for adding fluorine to a glass preform by exposing the glass preform to an atmosphere containing a fluorine compound gas and an inert gas such as He, and then consolidating the glass preform to make the glass preform transparent. As disclosed in PTL 3, in such a method for producing fluorine-containing silica glass in related art, each step is performed by setting pressure in a production apparatus to 1 atm or more.

PTL 4 discloses that a dehydration reaction of a glass fine particle aggregate is promoted by exposing the glass fine particle aggregate to an inert gas atmosphere containing halogen in a heating furnace in which pressure can be reduced to 0.1 torr or less. PTL 5 discloses a method for heating a glass preform while blowing a halogen gas such as chlorine under reduced pressure to make the glass preform transparent. PTL 6 describes a method for producing transparent glass, including a step of removing OH contained in a glass fine particle deposit by supplying a CO-containing gas in a heating furnace that can be vacuum deaerated, and then removing CO on a surface of the glass fine particle deposit under reduced pressure.

A method for producing fluorine-containing silica glass according to an aspect of the present disclosure includes:

In the methods disclosed in PTLs 1 to 3, as a size of a porous silica glass body (glass preform) increases, a time required for adding fluorine increases, and usage amounts of the fluorine compound gas and the inert gas also significantly increase. In particular, when He is used as the inert gas, an increase in a production cost due to the increase in the usage amount of the inert gas becomes a serious problem.

As in the methods disclosed in PTLs 4 to 6, when the dehydration reaction or transparent vitrification is performed in a vacuum container, usage amounts of an inert gas and the like in these steps can be reduced. However, PTLs 4 to 6 do not disclose a technique of adding fluorine to a porous silica glass body in the vacuum container, and do not disclose, for example, any condition for preventing deterioration of the vacuum container due to a fluorine compound and stably using the vacuum container for a long period of time.

An object of the present disclosure is to produce fluorine-containing silica glass with high productivity while preventing deterioration of a container used for production and reducing a usage amount of an inert gas.

According to a configuration of the disclosure described above, fluorine-containing silica glass can be produced with high productivity while preventing deterioration of a container used for production and reducing a usage amount of an inert gas.

First, embodiments of the present disclosure will be listed and described.

A method for producing fluorine-containing silica glass according to an aspect of the present disclosure includes:

According to a configuration, the fluorine-containing silica glass can be produced with high productivity while preventing deterioration of a container used for production and reducing a usage amount of an inert gas. Specifically, since moisture and an OH group in the porous silica glass body which reacts with the fluorine compound gas to generate an HF gas are desorbed in the degasification step before the fluorine addition step, the deterioration of the container can be prevented. Since the fluorine compound gas easily permeates into the porous silica glass body under reduced pressure, an addition rate of a fluorine compound is improved, and productivity is increased. Furthermore, since each of the above steps is performed under reduced pressure, it is not necessary to use the inert gas, and even if the inert gas is used, the usage amount of the inert gas can be reduced.

The term “under reduced pressure” refers to a state in which pressure in the furnace core tube is lower than atmospheric pressure. The temperatures of the degasification step and the fluorine addition step indicate a temperature of the furnace core tube surface in each of the steps.

In the production method, the degasification step is preferably performed at a temperature of 600° C. or more and 1200° C. or less.

According to this configuration, since the degasification step is performed at a temperature of 600° C. or more, the desorption of the OH group present as a silanol group can be further promoted. Since the degasification step is performed at a temperature of 1200° C. or less, a situation in which consolidation of the porous silica glass body proceeds and density increases is unlikely to occur. As a result, a situation in which the fluorine compound gas is difficult to permeate into the porous silica glass body in the fluorine addition step can be prevented.

In the production method, a maximum temperature in the degasification step is preferably 900° C. or more and 1200° C. or less.

According to this configuration, since a desorption reaction of the moisture and the OH group proceeds faster as the temperature is higher, the moisture and the OH group in the porous silica glass body can be efficiently desorbed.

In the production method, the degasification step preferably has a heating time of at least 30 minutes at the maximum temperature.

According to this configuration, the moisture and the OH group in the porous silica glass body can be more efficiently desorbed.

In the production method, an ultimate pressure at an end of the degasification step is preferably less than 500 pascals.

According to this configuration, since the degasification step is performed under sufficiently low pressure, the moisture and the OH group in the porous silica glass body can be more efficiently desorbed.

The term “pressure” used herein refers to the pressure in the furnace core tube.

In the production method, in the degasification step, an inert gas may be supplied into the furnace core tube.

According to this configuration, an impurity such as the moisture desorbed from the porous silica glass body can be efficiently exhausted to an outside of the furnace core tube.

In the production method, the fluorine compound gas used in the fluorine addition step is a compound gas of a group 14 element and fluorine, and preferably does not contain a chlorine atom or a hydrogen atom.

According to this configuration, corrosion of a metal component of the container when the fluorine compound gas leaks out of the furnace core tube can be further reduced.

In the production method, in the fluorine addition step, the fluorine compound gas may be diluted with an inert gas at a concentration of 1% or more and less than 100% and supplied, or the fluorine compound gas may be supplied at a concentration of 100%.

According to this configuration, the pressure in the furnace core tube can be easily controlled by adjusting the concentration of the fluorine compound gas.

In the production method, in the fluorine addition step, the heat-treating is preferably performed after stopping exhaustion of the furnace core tube.

According to this configuration, the fluorine compound gas is efficiently permeated into the porous silica glass body and a yield of the fluorine compound gas is improved.

In the production method, in the transparent vitrification step, it is preferable to further perform an evacuation treatment in which pressure is reduced while supplying an inert gas into the furnace core tube.

According to this configuration, the fluorine compound gas remaining in the furnace core tube or the container can be efficiently exhausted, and adsorption of the impurity into the furnace core tube or the container can be prevented.

In the production method, at an end of the transparent vitrification step, an ultimate pressure is preferably less than 500 pascals.

According to this configuration, for example, residual air bubbles in the fluorine-containing silica glass obtained by making the fluorine-containing silica glass transparent are reduced.

Hereinafter, an example of an embodiment of a method for producing fluorine-containing silica glass according to the present disclosure will be described with reference to the drawings. In the present specification, directions such as an upper side and a lower side may be referred to, but these directions are relative directions set for convenience of a description.

(Production Apparatus of Fluorine-Containing Silica Glass)

is a schematic view showing an example of a production of apparatus of the fluorine-containing silica glass. A production apparatusshown inincludes a container, a furnace core tube, a furnace core tube gas supply portion, a furnace body gas supply portion, a furnace core tube exhaust pipe, a furnace core tube exhaust valve, a furnace body exhaust pipe, a furnace body exhaust valve, a front chamber exhaust pipe, a front chamber exhaust valve, a vacuum pump, and an exhaust valve.

The containeris an airtight container. The containerincludes a front chamber, a rod, a gate valve, a furnace body, a heater, and a heat insulating material. The furnace core tubeis disposed inside the furnace body.

An insertion hole through which the rodis inserted is provided in an upper end portion of the front chamber. The rodis inserted into the front chamberthrough the insertion hole. An upper lid of the furnace core tubeis engaged with a lower end portion of the rod, and a porous silica glass body M is held on a further lower side thereof. The rodis connected to a lifting device (not shown) and can be lifted and lowered, for example, on a central axis of the furnace core tube.

The front chamberand the furnace bodyare made of, for example, a metal such as steel use stainless (SUS). When the production apparatusis not in use, an opening between the front chamberand the furnace bodyis closed by the gate valve. When the production apparatusis in use, the gate valveis opened to form the opening between the front chamberand the furnace body. Further, the porous silica glass body M held by the rodin the front chamberdescends and is inserted into the furnace core tubein the furnace body.

The heateris disposed around the furnace core tube. The heateris, for example, a resistance heating type heater. The heat insulating materialis disposed between the heaterand the furnace body.

The furnace core tubehas an opening on an upper side. The opening is closed by the upper lid engaged with the rodwhen the porous silica glass body M is inserted into the furnace core tube. The furnace core tubeis in an airtight state when the opening of the furnace core tubeis closed by the upper lid. The furnace core tubeis preferably made of carbon, for example, from the viewpoint of preventing deformation during high-temperature heating. In order to improve airtightness of the furnace core tube, it is also preferable to apply an airtight coating (for example, pyrolytic carbon, glassy carbon, silicon carbide, silicon nitride, or the like) to a surface of a material of the furnace core tube.

The furnace core tube gas supply portionsupplies a fluorine compound gas (for example, CFand SiF) or an inert gas (for example, Nand He) into the furnace core tube. The furnace body gas supply portionsupplies the inert gas into the furnace body. By controlling gas supply amounts from the furnace core tube gas supply portionand the furnace body gas supply portion, pressure inside the furnace core tubeis controlled and exhaustion of unnecessary components and the like to the furnace core tube exhaust pipeand the furnace body exhaust pipeis controlled.

The furnace core tube exhaust pipeis a pipe for exhausting an inside of the furnace core tube. The furnace body exhaust pipeis a pipe for exhausting an inside of the furnace body. The furnace core tube exhaust pipeis provided with the furnace core tube exhaust valve. The furnace body exhaust pipeis provided with the furnace body exhaust valve. Exhaust in the furnace bodyand the furnace core tubeis also controlled by these valves. The front chamber exhaust pipeis a pipe for exhausting an inside of the front chamber. The front chamber exhaust pipeis provided with the front chamber exhaust valve. Exhaust in the front chamberis controlled by the front chamber exhaust valve.

The furnace core tube exhaust pipe, the furnace body exhaust pipe, and the front chamber exhaust pipemerge at a downstream side. The vacuum pumpand the exhaust valveare provided in a pipe downstream of a merging point of these exhaust pipes. The vacuum pumpis a pump that exhausts and depressurizes the inside of the furnace body, the inside of the furnace core tube, and the inside of the front chamber. The exhaust valveis opened when exhaustion is performed. An exhaust treatment is performed downstream of the exhaust valve.

When the fluorine compound gas is contained in an exhaust gas, for example, the exhaust gas is sent to a cleaning tower. When the fluorine compound gas is not contained in the exhaust gas, for example, the exhaust gas is released into an atmosphere. The exhaust gas from the front chamberis released into, for example, the atmosphere. A destination of the exhaust gas is controlled by, for example, an opening/closing valve (not shown) provided downstream of the vacuum pump.

In the production apparatus, although the furnace core tubehas the airtightness, since it is difficult to make the furnace core tubecompletely airtight, a part of the gas supplied into the furnace core tubemay flow into the furnace body.

(Method for Producing Fluorine-Containing Silica Glass)

Hereinafter, a method for producing fluorine-containing silica glass according to the present embodiment will be described. In the production method according to the present embodiment, the fluorine-containing silica glass is produced using the production apparatusdescribed above.

The method for producing fluorine-containing silica glass according to the present embodiment includes: